Process of preparing high purity nickel



United States Patent PROCESS OF PREPARIN G HIGH PURITY NICKEL James H. Moore, Swampscott, Philip J. Clough, Reading, and Arthur E. Franks, Danvers, Mass, assignors to National Research Corporation, Cambridge, Mass., a corporation of Massachusetts No Drawing. Application November 2, 1953, Serial No. 389,868

4 Claims. (CI. 75-82) This invention is directed to the production of metals and more particularly to the production of high purity nickel and nickel-base alloys.

A principal object of thepresent invention is to provide improved processes for obtaining high purity nickel and nickel-base alloys.

Other objects will in part be obvious and will in part appear hereinafter.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description.

Recent developments in electronics and other fields, such as aeronautical engineering, have increased the demand for high purity nickel and nickel-base alloys. In the electronic field, high purity nickel is becoming of particular importance in applications such as the manufacture of cathodes which are coated with an oxide. In such uses, the control of the purity content of the nickel is extremely important. In commercial practices, deoxidation is performed by metallic deoxidizing agents, particularly silicon and magnesium. Consequently, these elements are present in residual amounts in the commercial nickel and often have a detrimental effect on certain electronic properties. An excessive amount of carbon and oxygen in nickel to be used in electronic tubes may also lead to poor performance due to evolution of gaseous carbon monoxide. Likewise, volatile elements, which may sublime from the nickel, give rise to detrimental deposits within the tube. Similarly, in the manufacture of nickel-base alloys, it is often extremely important to provide pure nickel starting material so that the resultant alloy is free of stringers of oxides and nitrides.

In melting nickel in a vacuum furnace, it has been found that the simple application of a vacuum on the order of .1 micron Hg abs. will result in improvement in the metal if the total molten charge is quite small, on the order of 15 to 30 pounds. With these small charges, it is relatively simple to reduce the carbon content to below .005 by reaction with oxygen in the molten bath to form carbon monoxide. This treatment will also reduce the nitrogen and hydrogen to very low values. Oxygen in the melt will also be decreased substantially since the dissociation pressure of nickel oxide at 1550 C. is on the order of one-third mm. Hg abs. Below that pressure, the solubility of oxygen varies as the square root of the pressure of oxygen above the melt. Thus, it appears possible to reduce the oxygen content to 005% or somewhat less by a simple vacuum treatment. However, as the melt size increases, such a method of elimination of oxygen becomes less desirable because of the considerable amount of gas which must be pumped out at relatively low pressure.

The present invention is primarily directed to improvements in vacuum refining techniques Which are economical and applicable to the treating of large batches of molten nickel. In the present invention, there are essentially three stages. The first of these is primarily a decarburization step but also results in the removal of most of the nitrogen and hydrogen. The second stage is primarily concerned with the deoxidation of residual oxygen by means of hydrogen reduction. The third stage is primarily a vacuum treatment for the removal of dissolved hydrogen.

In a preferred method of practicing the invention, the charge of nickel, for example 300 pounds, is placed in a magnesia crucible in a vacuum furnace and heated to a temperature of 1550" C. During this heating, the evolved gases are continuously pumped out of the vacuum furnace. The elimination of carbon is considered to be substantially complete when the rate of evolution of carbon monoxide has dropped to less than about 5 micron cubic feet per minute per pound of molten nickel. At this point, the pressure in the vacuum furnace is on the order of 1 to 10 microns Hg abs.

The deoxidation stage is then begun by the impingement of a stream of dry hydrogen on the surface of the melt with suificient velocity to form a crater in the molten nickel surface. The flow rate of hydrogen is preferably controlled to provide a minimum of .15 cubic feet (at the furnace pressure) per minute per pound of nickel. During this treatment, the pumping system is throttled to maintain a tank pressure at about 8 to 10 mm. Hg abs. The period of time required for the hydrogen deoxidation treatment is dependent upon the amount of oxygen in the charge when melted. This period is on the order of 20 to 30 minutes when the nickel charge comprises 300 pounds of electrolytic or carbonyl nickel. At the conclusion of this deoxidation period, the introduction of hydrogen is stopped and the furnace is again evacuated per pound of molten nickel.

The melt, after the above treatment, may be poured at this point, or, if alloying elements are to be added, the pressure in the furnace may be increased, if desired, by introducing a quantity of carefully purged inert gas. This is particularly desirable when the alloying constituents, such as aluminum and the like, are relatively volatile at the temperature of the molten nickel.

In the procedure described above, pure nickel, particularly suitable as a passive nickel for electronic tube use, is obtained from electrolytic nickel stock, the final nickel having a minimum nickel plus cobalt content of 99.89%. The maximum content of impurities in the refined nickel is as follows:

This may be compared with the commercial product from the normal melting of electrolytic nickel which has This. rate is on the order of 5 micron cubic feet per minute 99.00% minimum nickel plus cobalt content with the following impurities-z The ordinary commercial nickel will have relatively large quantities of oxygen and nitrogen. These make the nickel unsuited for specialized electronic applications or for the manufacture of high purity alloys for high temperature use.

The application of this basic purification technique to a dilute nickel a'lloy may be illustrated in the case of a so-called normal cathode alloy. The emission from the coating? of such an alloy appears to depend greatly upon the small amounts of certain elements present in the nickel. In the commercial product which has been used for this application, only occasional melts have been accepted, and even within the material taken from a single melt, at considerable variation in properties has existed.

However, it hasbeen found that such a variation in properties can be eliminated by the production of such an alloy in vacuum. A melt of high purity nickel, as heretofore described, is first made. At the conclusion of the refining period, an inert gas pressure on the order of 50 mm; Hg abs; is introduced into the vacuum furnace, and the analysis of the melt adjusted so that certain elements are at a higher level than in the pure or passive type of nickel.

The analysis of this normal cathode alloy, as obtained by the above-described vacuum melting methods and duplicated from melt to melt, is as follows:

When melts of such an alloy are produced by this vacuum technique, not only are the general emission characteristics improved but also uniformity is exhibited from melt to melt and throughout an individual melt.

Since certain changes may be made in the above process without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. The process of preparing high purity nickel for use as a cathode material in an electron tube and the process comprising the steps of melting nickel in a vacuum furnace, maintaining said molten nickel under a low pressure on the order of a few microns Hg abs. until the gas evolution from the nickel drops to a rate less than about 5 micron cubic feet per minute per pound of nickel to reduce the carbon content of the nickel to less than 005%, thereafter impinging a stream of hydrogen on the surface of the molten nickel, the rate of hydrogen impingement being on the order of at least 15 cubic feet of hydrogen (at the furnace pressure) per minute per pound of nickel, maintaining the furnace pressure at about a few mm. Hg abs. during the hydrogen impingement and continuing the hydrogen impingement until the total oxygen content of the nickel is less than about 0.005 and thereafter lowering the furnace pressure to a few microns Hg abs. to remove hydrogen from the nickel.

2. The process of claim 1 wherein the pressure in the furnace is maintained at about 8 to 10 mm. Hg abs. during the hydrogen deoxidation of the nickel.

3. The process of claim 1 wherein the nickel is alloyed with other metals after the removal of hydrogen from the melt.

4. The process of claim 1 wherein a cathode alloy is prepared by introducing into the furnace a partial pressure of inert gas on the order of 50 mm. Hg abs. and maintaining said partial pressure of inert gas while adding to said molten nickel sufiicient alloying elements to yield an alloy containing on the order of 0.04 to 0.07% carbon, 0.02 to' 0.05% silicon, 0.05 to 0.10% iron, 0.03 to 0.07% magnesium, 0.01 to 0.03% titanium, 0.5 to 0.6% columbium, up to 0.03% copper, up to 0.005% aluminum, up to 0.001% chromium, up to 0.001% sulfur, and up to 0.001% oxygen.

OTHER REFERENCES The Journal of the Iron and Steel Institute, vol. 162, page 325, published 1949. 

1. THE PROCESS OF PREPARING HIGH PURITY NICKEL FOR USE AS A CATHODE MATERIAL IN AN ELECTRON TUBE AND THE PROCESS COMPRISING THE STEPS OF MELTING NICKEL IN A VACUUM FURNACE, MAINTAINING SAID MOLTEN NICKEL UNDER A LOW PRESSURE ON THE ORDER OF A FEW MICRONS HG ABS. UNTIL THE GAS EVOLUTION FROM THE NICKEL DROPS TO A RATE LESS THAN ABOUT 5 MICRON CUBIC FEET PER MINUTE PER POUND OF NICKEL TO REDUCE THE CARBON CONTENT OF THE NICKEL TO LESS THAN .005%, THEREAFTER IMPINGING A STREAM OF HYDROGEN ON THE SURFACE OF THE MOLTEN NICKEL, THE RATE OF HYDROGEN IMPINGEMENT BEING ON THE ORDER OF AT LEAST 15 CUBIC FEET OF HYDROGEN (AT THE FURNACE PRESSURE) PER MINUTES PER POUND OF NICKEL, MAINTAINING THE FURNACE PRESSURE AT ABOUT A FEW MM. HG ABS. DURING THE HYDROGEN IMPINGEMENT AND CONTINUING THE HYDROGEN IMPINGEMENT UNTIL THE TOTAL OXYGEN CONTENT OF THE NICKEL IS LESS THAN ABOUT 0.005%, AND THEREAFTER LOWERING THE FURNACE PRESSURE TO A FEW MICRONS HG ABS. TO REMOVE HYDROGEN FROM THE NICKEL. 